<?php

$options = cartesian(expand_pattern('a..z0..9'), 2);
print_r($options);

function expand_pattern($pattern) {
    $bias = 0;
    $flags = PREG_SET_ORDER | PREG_OFFSET_CAPTURE;
    preg_match_all('/(.)\.\.(.)/', $pattern, $matches, $flags);
    foreach ($matches as $match) {
        $range = implode('', range($match[1][0], $match[2][0]));
        $pattern = substr_replace(
            $pattern, 
            $range, 
            $bias + $match[1][1],
            $match[2][1] - $match[1][1] + 1);
        $bias += strlen($range) - 4;
    }

    return $pattern;
}

function cartesian($pattern, $length) {
    $choices = strlen($pattern);
    $indexes = array_fill(0, $length, 0);
    $results = array();
    $resets = 0;

    while ($resets != $length) {
        $result = '';
        for ($i = 0; $i < $length; ++$i) {
            $result .= $pattern[$indexes[$i]];
        }
        $results[] = $result;

        $resets = 0;
        for ($i = $length - 1; $i >= 0 && ++$indexes[$i] == $choices; --$i) {
            $indexes[$i] = 0;
            ++$resets;
        }
    }

    return $results;
}